This application relates to estimation of radio frequency propagation time (“time of flight”) between radio transceivers, and more particularly relates to sub-decimeter range estimation between two wireless local area network (“WiFi”) transceivers.
Radio frequency transmissions propagate at approximately 3×108 meters per second. Therefore, in order to use a propagation time to estimate a propagation distance to an accuracy of 0.1 meters (a decimeter), the time must be known to an accuracy of greater than 0.3 nanoseconds. That is, sub-nanosecond accuracy is necessary for such an approach to distance estimation.
Prior approaches have been presented to resolve time of flight to approximately 10 nanoseconds using the clocks of WiFi cards or other related methods. However, the accuracy of such approaches is insufficient for decimeter range accuracy.
One use of propagation time or distance estimation is localization. Others have attempted to use WiFi transceivers for sub-meter accuracy of localization. However, such prior approaches have not directly measured propagation time or distance between transceivers. For example, multiple reference transceivers are needed to infer a location of a target transceiver using differences in propagation time between different reference transceivers and a transceiver at an unknown location, rather than absolute propagation times, may be used to infer a location of a target transceiver. For example, such differences in propagation time may be used to determine direction of arrival, which in turn is used to determine the target location by triangulation.
State-of-the-art systems may achieve an accuracy of tens of centimeters, even using commodity WiFi chipsets. Such systems generally target enterprise networks, where multiple WiFi access points can combine their information and cooperate together to locate a target device. However, the vast majority of homes and small businesses today have a single WiFi access point and therefore such multiple-access-point approaches are not applicable.
There exist non-WiFi systems that can accurately measure the absolute time of flight, and hence localize using a single receiver. Such systems generally use specialized ultra wideband radios that span multiple giga-Hertz. Because time resolution is inversely related to the radio bandwidth, such devices can measure time of flight at sub-nanosecond accuracy, and hence localize an object to within tens of centimeters. In contrast, directly measuring time with a 20 or 40 mega-Hertz bandwidth equivalent to a WiFi radio transmission would result in errors of 7 to 15 meters using such techniques.
Prior work has failed to recognize and/or provide effective solutions to overcome effects that make it difficult to make accurate over-the-air propagation time estimates between two WiFi transceivers. These effects include:                Limited transmission bandwidth        Carrier frequency offset between transceivers        Unknown packet detection delay at receivers        Sampling frequency offset between transceivers        Multiple propagation paths between transceivers        
There is a need to improve available technology for distance estimation, and in particular to overcome one or more of these effects to achieve improved time and/or distance accuracy.